The "Flameproof Enclosure" ignition protection type is a type of ignition protection, in which electric operating equipment and other appliances are accommodated in a closed housing. The housing must satisfy specific test conditions. It is necessary, inter alia, to ensure that, if an ignitable gas mixture ignites in the housing, no particles and no hot gases can escape outward which may possibly ignite the atmosphere in the vicinity of the housing, if an ignitable gas mixture is also present there. This condition relating to the retention of ignitable particles and hot gases can be achieved if the cover of the housing forms with the housing a so-called "ex-gap". During passage through the "ex-gap", the particles and gases are sufficiently cooled to be no longer capable of causing any damage outside the housing.
Housings manufactured from metal, which were machined by cutting in the region of the "ex-gap" have hitherto been used for this purpose. A housing body or a housing blank which, in the broadest sense, is bowl-shaped is produced by means of a diecasting technique, by the sandcasting method or by other forming techniques. A cover produced by the same technique is placed onto the free edge of this housing body. The housing parts, that is to say both the housing body and the cover, produced by means of the forming techniques are so rough and uneven in the region of those faces which lie on one another at the joining plane when the housing is screwed together that they first have to be fine-machined on these faces by cutting machining, so that the gap established in the parting plane satisfies the conditions demanded of an "ex-gap".
The same also applies accordingly to housings which are welded together from steel parts.
So that the cost of manufacturing such metal housings could be in any way at all kept under control, the gap necessarily had to be a planar gap. It is not simple even to make a planar gap, because the relatively large and comparatively thin-walled parts are easily distorted in the machine tool during the chucking operation. On the other hand, a "tongue-and-groove joint" is virtually ruled out. Although it may still be relatively simple to mill a groove by means of an end-milling cutter, it would be unreasonably expensive to produce a tongue or rib fitting into this groove with a negligible gap width.
The fact that only planar gaps could be produced in any way cost-effectively and that the gap occupies a certain width in the direction of the gas flowing out in the event of ignition resulted in the widths at the joining point between the housing body and cover which had an adverse influence on the housing dimensions.